The present invention is related to electric machines, and in particular to electric machines utilizing a flux-regulated permanent magnet rotor.
In the simplest terms, generators convert mechanical energy to electrical energy via the interaction of rotating magnetic fields and coils of wire, wherein motors convert electrical energy to mechanical energy via the interaction of magnetic fields. An electric machine may therefore operate as either a generator or a motor depending on the direction of power flow (e.g., mechanical to electrical or electrical to mechanical). A multitude of electric machine architectures have been developed with various means of providing interaction between magnetic fields and coils of wire. For example, a permanent magnet machine (PMM), operating in a generating mode, utilizes permanent magnets to generate a constant magnetic field, which is rotated via the mechanical energy supplied by a prime mover such that the rotating magnetic field interacts with the stator coils to provide an output voltage. Another type of electric machine, again operating in a generating mode, supplies current through a coil to generate the desired magnetic field on the rotor, which is rotated via the mechanical energy supplied by a prime mover, such that a rotating magnetic field is created that interacts with the stationary stator coils to provide an output voltage.
In the former example, the output voltage supplied by the PMG depends only on the magnitude of the mechanical energy supplied by the prime mover. In the latter example, the output voltage of the generator can be regulated by varying the current supplied to the exciter coil. For applications in which the output voltage must be regulated, the latter example, known as a wound field synchronous machine, is widely utilized. However, permanent magnets offer advantages over excitation provided via an exciter winding, including improved efficiency over more traditional wound field synchronous machines. The key drawback to permanent magnets is that magnetic flux provided by the permanent magnets is constant, unlike an exciter coil in which the magnetic flux can be regulated by increasing or decreasing the current through the exciter coil. Permanent magnets would therefore be useful in a number of generator applications if a form of regulation could be introduced.